Abstract About 20 million people worldwide are chronically infected with hepatitis delta virus (HDV) that is a natural sub- viral agent of hepatitis B virus (HBV). HDV takes only the envelope proteins (i.e. surface antigen, HBsAg) from HBV to form its virions and infect hepatocytes via HBV receptor. HDV can accelerate liver disease, cause more fast/frequent cirrhosis, and influence liver carcinogenesis. Interferon treatment, the only approved anti-HDV therapy, is unsatisfactory. Anti-HBV drugs do not block HDV infection. No drugs exist in clinical practice that target HDV directly. Mechanisms mediating transient or chronic HDV infection are poorly understood. The key preliminary data for this study were obtained by using woodchucks (WCs) chronically infected with HBV-related woodchuck hepatitis virus (WHV), which were also super-infected with HDV. This model is an invaluable experimental system for studying HDV infection/pathogenesis, anti-HDV host response and testing anti-HDV interventions. We found for the first time that HDV clearance could be associated with fragmentation of virion- associated HDV RNA genomes, which causes the loss of replication capacity of HDV genomes that in turn leads to the loss of HDV infectivity. On the other hand, the progression to chronicity apparently would require sufficiently high replication capacity and infectivity, which likely support continuous HDV spread and super- infection ensuring HDV persistence. We also found that the fraction of serum HDV bound to antibodies against WHV surface antigen (or anti-envelope antibodies, Abs) was always very small and lower than the fraction Ab- WHV complexes, which could suggest that such absence of Ab-mediated antiviral defense may at least in part be linked to high rates of HDV chronicity caused by the super-infection. This proposal will determine if HDV infection in humans is regulated by similar mechanisms. We will analyze serum samples from individuals that either cleared HDV or became chronic HBV/HDV carriers. Aim 1 will find if HDV clearance is associated with (i) fragmentation of HDV genomes, (ii) loss of the replication capacity of virion-associated HDV genomes, and (iii) loss of HDV virions' infectivity, while progression to chronicity is linked to sufficiently high genome replication capacity and virions' infectivity along with substantial fraction of intact full-length HDV RNA genomes in the virions. We will also find if there is another mechanism(s) of HDV clearance that is not linked to alteration of HDV properties. Aim 2 will find if the fraction of Ab-HDV complexes is always lower than the fraction of Ab-HBV complexes, thus suggesting the differences in regulation of HBV and HDV infections by anti-HBsAg Abs. Aim 2 will also determine if Abs induced by immunization with HBsAg protect against HBV and HDV by binding both viruses equally well, or Abs efficiently bind only HBV, and thus protect against HDV indirectly via preventing (i) the establishment of HBV reservoir in infected livers and (ii) continuous production of HBsAg that is needed for HDV persistence. Overall, our studies will advance the understanding of mechanisms regulating the outcomes of HDV infection, and will find whether HDV infection is regulated by similar mechanisms in humans and WCs.